Mass spectrometers are instruments used to analyze the mass and abundance of various chemical components in a sample. Mass spectrometers work by ionizing the molecules of a chemical sample, separating the resulting ions according to their mass-charge ratios (m/z), and then counting the number of ions at each m/z value. The resulting spectrum reveals the relative amounts of the various chemical components in the sample.
One type of mass analyzer used for mass spectrometry is called a quadrupole ion trap. Quadrupole ion traps take several forms, including, but not limited to, three-dimensional ion traps, linear ion traps, rectilinear ion traps, toroidal ion traps, planar ion traps and cylindrical ion traps. The operation in all cases, however, remains essentially the same. DC and time-varying radio frequency (RF) electric signals are applied to the electrode(s) to create electric fields within the ion trap. Those fields trap ions within the central volume of the ion trap. Then, by manipulating the amplitude or frequency of the electric fields, ions are selectively ejected from the ion trap in accordance with their m/z.
An ion detector records the number of ejected ions at each m/z as the ions arrive. A common type of ion detector is the channel electron multiplier (OEM). By cascading multiple secondary emissions of electrons, a OEM amplifies the original ion impact. Although the OEM can typically amplify the current by a factor of up to 107, further amplification is necessary. For instance, ten singly charged ions being ejected from the ion trap per second creates a current of 1.6×10−18 A. The OEM raises this current to 1.6×10−11 A, which is still too small to output as a signal outside the instrument without amplification. Even for larger samples consisting of thousands of ions, the output signal is too small to be used as a reliable measurement. Thus, the output of the ion detector is typically increased by a conventional electronic amplifier. Finally, a computing device, such as a digital signal processor, field programmable gate array (FPGA) or a microprocessor, processes the resulting signal to produce a mass spectrum, or a chart showing the relative abundances of measured ions for each m/z.
Mass spectrometer designs are optimized for a combination of speed, sensitivity, and resolution depending on the particular application. A common approach to improving the sensitivity, or the minimum detection level for a particular chemical in a target sample, is to increase the number of ionized molecules. However, in samples consisting of two or more chemicals where one chemical is far more prevalent than the other(s), increasing the total number of ions may saturate the measurement of the most abundant chemical, thus reducing the dynamic range of the instrument, or the range of sample concentrations that can be measured. In addition, many applications require the detection of trace amounts of chemical which means that only a small number of molecules are available to be ionized and analyzed by the mass spectrometer, regardless of the relative amount of chemicals in the sample.
Another approach to improving sensitivity is by reducing the noise, or the unwanted components of the mass spectrum contributed by factors other than the detection of target ions. There are two different types of noise present in mass spectrometry. One is chemical noise, which may result from chemical contamination of the ion trap or ion detector, molecules being ionized but not being ejected to the detector at the appropriate time, or other means that result in detection of particles other than sample ions. The second is electrical noise either generated in or picked up by the ion detection circuitry. These noise sources produce a noise floor observed on the resulting mass spectrum. Any spectral peak corresponding to the presence of a compound that has an intensity less than this noise floor, or within a small ratio of signal to noise would not be detected. By reducing the noise level, the signal to noise ratio is improved, which in turn increases the probability of detecting small signals and also expands the dynamic range of the instrument.
A persistent mass spectrum electrical noise source is the high-voltage RF signal applied to the ion trap electrode(s), which couples to the ion detector amplifier. This source of noise is particularly troublesome in compact or miniaturized systems where shielding between the high-voltage components and the ion detection circuitry is minimal. Conventional filtering techniques may use additional signal processing circuitry or software to remove noise components from the ion detector amplifier output. However, several issues prevent such techniques from adequately removing the RF signal. First, the high-voltage RF signal may be similar in frequency content to the mass spectral output signal of the ion detector amplifier. Furthermore, the amplitude variation of the RF signal depends on several parameters, including the mass range and local temperature, and is therefore difficult for filter designers to model accurately. It also adds extra processing time which may adversely affect the performance of a fast instrument.
Alternatively, if the phase of the RF signal is set to be non-synchronous to the start of the spectrum scan for several repeated scans of a chemical sample, the RF signal noise can be averaged away over time, but this requires a large number of spectra to be averaged together in order to reduce the RF noise sufficiently. In applications where speed is important, such a method would take too much time. Furthermore, when detecting fast transient chemical signals, which may only exist in a small number of spectral scans before disappearing, averaging away the noise will also average away the desired signal.
Thus, there is a need for an RF noise cancellation system that does not affect the spectral signal and can be performed in real time with a minimal number of scans.